1. Field of the Invention
The present invention relates to hydroprocessing systems and methods, in particular for efficient reduction of catalyst-fouling aromatic nitrogen components in a hydrocarbon mixture.
2. Description of Related Art
Hydrocracking operations are used commercially in a large number of petroleum refineries. They are used to process a variety of feeds boiling in the range of 370° C. to 520° C. in conventional hydrocracking units and boiling at 520° C. and above in the residue hydrocracking units. In general, hydrocracking processes split the molecules of the feed into smaller, i.e., lighter, molecules having higher average volatility and economic value. Additionally, hydrocracking typically improves the quality of the hydrocarbon feedstock by increasing the hydrogen to carbon ratio and by removing organosulfur and organonitrogen compounds. The significant economic benefit derived from hydrocracking operations has resulted in substantial development of process improvements and more active catalysts.
Mild hydrocracking or single stage hydrocracking operations, typically the simplest of the known hydrocracking configurations, occur at operating conditions that are more severe than typical hydrotreating, and less severe than typical full pressure hydrocracking. Single or multiple catalysts systems can be used depending upon the feedstock processed and product specifications. Multiple catalyst systems can be deployed as a stacked-bed configuration or in multiple reactors. Mild hydrocracking operations are generally more cost effective, but typically result in both a lower yield and reduced quality of mid-distillate product as compared to full pressure hydrocracking operations.
In a series-flow configuration the entire hydrocracked product stream from the first reaction zone, including light gases (e.g., C1-C4, H2S, NH3) and all remaining hydrocarbons, are sent to the second reaction zone. In two-stage configurations the feedstock is refined by passing it over a hydrotreating catalyst bed in the first reaction zone. The effluents are passed to a fractionating zone column to separate the light gases, naphtha and diesel products boiling in the temperature range of 36° C. to 370° C. The hydrocarbons boiling above 370° C. are then passed to the second reaction zone for additional cracking.
Conventionally, most hydrocracking processes that are implemented for production of middle distillates and other valuable fractions retain aromatics, e.g., boiling in the range of about 180° C. to 370° C. Aromatics boiling higher than the middle distillate range are also included and produced in the heavier fractions.
In all of the above-described hydrocracking process configurations, cracked products, along with partially cracked and unconverted hydrocarbons, are passed to a distillation column for fractionating into products including naphtha, jet fuel/kerosene and diesel boiling in the nominal ranges of 36° C.-180° C., 180° C.-240° C. and 240° C.-370° C., respectively, and unconverted products boiling in the nominal range of above 370° C. Typical jet fuel/kerosene fractions (i.e., smoke point>25 mm) and diesel fractions (i.e., cetane number>52) are of high quality and well above the worldwide transportation fuel specifications. Although the hydrocracking unit products have relatively low aromaticity, aromatics that do remain lower the key indicative properties (smoke point and cetane number) for these products.
A need remains in the industry for improvements in hydrocracking operations for heavy hydrocarbon feeds to produce clean transportation fuels in an economical and efficacious manner.